1. Field of the Invention
This invention relates generally to devices for applying sealant to structure defining a junction locus, e.g., the junction of a cable with another cable or a coupling with another member, where sealants are desirably employed to provide environmental protection to the junction, to provide shielding against electromagnetic interference (EMI) and/or radio frequency leakage, etc.
2. Description of The Related Art
In numerous applications where radio frequency (RF) devices are employed, e.g., impedance matching/coupling devices, ground protection devices (ground fault interruptor systems), radar transmitters and/or receivers, and signal splitting systems, the occurrence of EMI and RF leakage into or out of the device seriously adversely affect its operation for its intended purpose.
As a specific example, in cable television (CATV) systems, RF signals that leak into the ambient environment cause interference with other equipment containing electromagnetic components which operate within the same frequency range, such as aircraft telecommunications and guidance systems. As a consequence, the Federal Communication Commission (FCC) has imposed stringent rules and regulatory sanctions relating to signal leakage from cable systems. Conversely, the RF leakage of stray electromagnetic signals into the cable system is a significant concern, since such leakage causes interference with the transmitted cable signal, resulting in substandard reception by the CATV viewer receiving the transmitted signal at his television or monitor. Such interference resulting from RF leakage is reported to be the single largest maintenance problem facing the CATV industry today.
The main portal of RF signal leakage in CATV systems is loose or corroded couplings on the CATV cables, such as the drop connectors connected to port taps on the tap box in the main co-axial cable line serving an individual CATV subscriber, and the drop connectors connected to the ground block which is attached to the subscriber's building in which the CATV signal is processed for viewing.
General approaches which have evolved for reducing EMI or RF interference include shielding means, such as plate members and housings constructed of materials which absorb radio and other electromagnetic waves.
Such shielding elements are generally useful in attenuating EMI/RF interference in the signal processing portion of the system, but do not solve the problem of EMI or RF leakage associated with the junctions of such signal processing (or generating) units at their connections with external leads, such as electrical cables or wires, nor do they solve the problems associated with line-to-line couplings of the cables or wires which transmit electromagnetic signals into or out of the signal processing unit.
In some systems, "rubber boots" have been utilized over the couplings joining signal processing units and their external leads, to eliminate or substantially reduce EMI and RF leakage at such junctions. Such boots are similar in design to covers used in automotive systems for spark plug connector wires, and generally work well initially when properly fitted to cover the associated coupling.
Nonetheless, such rubber boots due to their generally high coefficient of friction on the interior surfaces mating with the coupling, frequently tend to be poorly installed, so that they incompletely cover the coupling, and allow EMI and RF leakage to adversely affect the operation of the system.
Further, where the system is subject in operation to vibration, translation, flexural stresses on the leads and/or signal processing unit, etc., the couplings even when overlaid with rubber boots tend to loosen, and thereby increase the susceptibility of the system to EMI and RF leakage.
In addition, when electromagnetic devices are employed in exterior environments, corrosion of couplings and associated elements of the system may result from the presence of atmospheric moisture. The corrosion may be accelerated by the presence of dissimilar metals in the couplings or adjacent portions of the system, with consequent adverse affect on the structural integrity as well as the performance characteristics of the system.
Such corrosive action is facilitated by poorly fitted or loosened boots. Although it is known in the art to coat couplings and the interior surfaces of boots employed thereover with silicone greases to serve as moisture permeation barriers, such greases dry out and crack or otherwise dissipate in use, so that their anti-corrosive efficacy is lost.
The foregoing problems are particularly severe in CATV junction boxes, which are typically employed in exterior environments on overhead or underground signal cables, and have a plurality of couplings joining the box to the signal cable and the service (output) cables. In such environments, the junction box/cable couplings readily tend to work loose, as well as corrode.
In co-pending U.S. application Ser. No. 07/003,873 filed Jan. 16, 1987 in the name of James P. Moran, Jr., there is disclosed a means for protecting the junction of signal processing means and electromagnetic transmission means from adverse operating effects such as EMI/signal loss, signal attenuation, and RF leakage, by disposing at the junction of the signal processing means and the electromagnetic transmission means a layer of a material which is blockingly effective against such adverse operating effects. The blockingly effective material has a volumetric resistivity of from about 10.sup.-3 to about 2,000 ohm-centimeters.
Specifically, the blockingly effective material disclosed in this patent application may comprise a binder, such as a nylon, rubber, or halogenated vinyl polymer, and an electrically conductive component, such as carbon, aluminum, nickel, copper, or silver, dispersed in the binder. A particularly preferred composition of such type is disclosed as comprising an ethylene-butylene-styrene block copolymer, commercially available from Shell Chemical Company as KRATON.RTM. G.1650, and 15% by weight, based on the dry solids of the formulation, of a conductive carbon powder having an average surface area (BET) of 2.54 meters.sup.2 /gram, in 1,1,-trichlorothane solvent solution.
This application further discloses that the solvent-based formulation described above may be applied to the junction locus of the electromagnetic device by dispensing the formulation in a non-aerated form from a pressurized container equipped with a nozzle which when manually pressed off-center, causes a rapid flow of the formulation in a non-aerated condition to be dispensed onto the cable junction.
The pressurized container dispensing of sealant materials onto the cable junction, as disclosed in the above-described Moran, Jr. patent application, is highly effective in reducing EMI and RF leakage effects to a virtually negligible level when the sealant composition is fully circumferentially applied to the cable junction.
Unfortunately, however, as already described, the CATV system frequently comprises an above-ground cable network with the cable being suspended on poles or otherwise disposed in an overhead position. It is then necessary for the installer or maintenance person to physically secure himself on the pole or structure from which the cable and cable junction box are suspended, and to apply the sealant material. In such position, it is frequently difficult to insure a circumferentially continuous sealing mass on the cable junction, particularly in inclement or windy weather conditions. As a result, the applied sealant is not continuous over the full areal extent of the coupling or junction locus, and this in turn provides a path for EMI and RF leakage, which the applied sealant is intended to eliminate or substantially reduce.
Accordingly, it would be a significant advance in the art to provide a means for applying a sealant material to a junction locus, such as a cable connection, which results in a circumferentially continuous coating of the sealing material over the full areal extent of the junction locus.
It is therefore an object of the present invention to provide a means for applying sealant material to a junction locus so that the resulting sealant coating is circumferentially continuous and covers the fully areal extent of the locus of the junction.
It is a further object of the invention to provide means of such type which are quickly and conveniently employed even in junction locations which are difficult to physically access, e.g., overhead CATV junction box fittings to which signal transmission cables are joined.
It is a further object of the invention to provide a sealant application means of such type, which may be readily manufactured at comparatively low cost.
Other objects and advantages of the invention will be more fully apparent from the ensuing disclosure and appended claims.